Hammer drills are power tools that can generally operate in three modes of operation. The hammer drill will have a tool bit that can be operated in a hammer mode, a rotary mode and a combined hammer and rotary mode. For the hammer and combined hammer and rotary mode, it is necessary to convert the rotary motion of the output shaft of the tool's motor into a reciprocating motion in order to power the hammering action.
A mechanism for converting the rotary motion of the output shaft of the motor into reciprocating motion is described in GB2038986. Referring to FIG. 1 which shows a partially cut away perspective view of a drive mechanism described in GB2038986, and to FIG. 2 which shows a cross sectional view of the drive mechanism of FIG. 1, a hollow piston 2 is slidably mounted in a sleeve 4 such that the hollow piston 2 can reciprocate relative to the sleeve 4. A ram (not shown) is slidably disposed in the hollow piston 2 in order to convert the reciprocation of the hollow piston two into a hammering action as will be known to persons skilled in the art.
A crank pin 6 connects the hollow piston 2 to a circular crank plate 8 and comprises a cylindrical head 16 which is slidably disposed in a bearing 10 disposed on the rear of a hollow piston 2. The crank pin 6 also comprises a spherical head 12 which is trapped in a spherical socket 14 disposed in the crank plate 8. The crank plate 8 is formed from two halves, 8a and 8b, which mate to define a spherical socket 14 for trapping the spherical head 12 therebetween.
As the crank plate 8 rotates, the crank pin 6 alternately pushes and pulls the hollow piston 2 forwardly and rearwardly such that the hollow piston 2 reciprocates within the sleeve 4. During the reciprocating motion of the hollow piston 2, the spherical head 12 of the crank pin 6 follows a circular path, whilst the cylindrical head 16 of the crank pin 6 slides up and down in bearing 10, as the bearing 10 and the hollow piston 2 rocks laterally from side to side. As a result of the shape of spherical socket 14, the spherical head 12 is trapped in the crank plate 8 in order to prevent the crank pin 6 from becoming either disengaged from the bearing 10, or the crank plate 8.
The above mechanism suffers from the drawback that the spherical head 12 of the crank pin 6 needs to be permanently attached to the crank plate 8. This means that either the spherical head must be press fitted into the crank plate such that it is in an interference fit or, as in the embodiment shown in FIGS. 1 and 2, the crank plate must be formed from a plurality of pieces that come together to form a part-spherical socket. These features both increase the cost and manufacturing complexity of the drive mechanism of GB2038986.
Preferred embodiments of the present invention seek to overcome the above disadvantages of the prior art.